In one type of commonly used particleboard manufacturing system, a moving conveyor continuously transports flat metal plates known as cauls past a series of formers which are supplied with wood particles that are impregnated with resin and wax. The formers deposit the wood particles on the moving cauls at a rate which is determined by two motor driven devices within each former, with the speed of each motor driven device being controllable by electrical signals. As the wood particles are deposited, the particles loosely adhere to one another and form a web that is automatically trimmed to form a mat of desired length and width as the cauls pass from beneath the formers.
As in all particleboard manufacturing systems, the weight of each mat determines some of the more important characteristics of the finished particleboard that is formed from that mat. Hence, there exists an acceptable weight range for each type and thickness of particleboard that is produced. Since the volume of the deposited wood particles, and hence the mat weight, is inversely proportional to the speed of the conveyor and is directly proportional to the rate at which the formers deposit the wood particles, it has long been the practice within the prior art to manually control each of the former speeds (rate at which the formers deposit wood particles) and to manually control the conveyor speed in an attempt to continuously produce mats of an acceptable weight.
Such manual control has not resulted in the efficient manufacture of particleboard largely because of the wide variation in density of the wood particles employed in the process. One reason for this variation in wood particle density is that a variety of different woods may be used with the types of wood or the mixture of different woods being determined by the type of wood available at any given time. Further, even in situations in which a single type of wood is utilized, variations in wood particle density are encountered. In addition, the density of the wood particles is further affected by conditions such as the moisture content of the wood particles, variations in the specific density of the resin being employed, and the ambient humidity and temperature. Thus, it can be recognized that rather abrupt changes in wood particle density can occur at any time and that each such disturbance to the manufacturing process necessitates prompt control action to maintain the weight of the mats being produced within the acceptable limits.
Manual control to compensate for disturbances in the particleboard manufacturing system is hindered by the fact that the weight of a particular mat is only known after several other mats have been deposited by the formers. This condition arises since the apparatus for forming the length of each mat is generally located between the last former and a scale upon which each mat is weighed as it passes to a press located at the terminus of the conveyor system. Thus, it can be appreciated that the system operator is not aware of a disturbance such as an abrupt change in wood particle density until a number of mats have been formed. Further, when the operator initiates a change in one or more former speeds and/or a change in the conveyor speed, the effect of this action cannot be observed until an empty caul passes beneath the formers and reaches the scale. Thus, even the most experienced and proficient operator of a prior art particleboard manufacturing system is not able to control the manufacturing system so as to achieve highly efficient production.
The problem of manually controlling the particleboard manufacturing system is further complicated in that several situations arise in which it is necessary to change the conveyor speed and/or the former speeds. First, in most particleboard manufacturing systems it is necessary to periodically set the conveyor speed to achieve a desired production rate. In order to continue producing mats of an acceptable weight during such a change in production rate, the operator must manually adjust the conveyor while adjusting the former speeds (either simultaneously of alternately effecting small changes in conveyor speed and former speeds) to continue producing acceptable mats. As in the case of continuously controlling mat weight at a single production rate, even the most proficient and experienced operator is often hard pressed to effect a desired change in production rate without producing a substantial number of unacceptable mats.
Another situation that arises to complicate the manual control of a prior art particleboard manufacturing system is changing from the production of one grade of particleboard to the production of another grade. Since each grade or type of particleboard can require a different mat length and a different mat width and each grade includes finished particleboard of various thickness, changing from the manufacture of one grade to the manufacture of another grade can call for substantial adjustment of the conveyor speed and each former speed. The grade change situation is further complicated in that in order to change the length and/or width of the mats being produced, the operator must make certain mechanical adjustments to other apparatus within the particleboard manufacturing system. Further, when a change is made to begin manufacturing a different type of particleboard, the operator will often be required to also adjust the rate at which the new grade of particleboard is being produced. Hence, with respect to prior art particleboard manufacturing systems, changing from the production of one grade of particleboard to the production of another grade generally produces a substantial number of unacceptable mats.
One further condition that arises to hinder efficient operation of a prior art particleboard manufacturing system occurs when there is a short interruption in the operation of the manufacturing system. Such an interruption can occur, for example, when the manufacturing system must be shut down for a short interval to perform maintenance duties, or a power interruption occurs. The problem presented by such a production interruption arises because the wood particles contained within the formers begin to lose moisture content and hence effectively become less dense. When production is resumed, such wood particles pass more readily from the formers and unless the former speeds are adjusted, the weight of each mat will increase above that being produced when the shutdown commenced. Since the decrease in moisture content is dependent on the duration of the shutdown, the moisture content of the wood particles prior to the shutdown, and is also dependent on other factors such as the ambient temperature and humidity, it is difficult for the system operator to manually adjust the former speeds to properly compensate for this condition. Further, since the moisture content of the wood particles will generally increase once the supply of particles held within the formers during the production interruption is exhausted, the operator will again be called upon to adjust the former speeds.
Thus, it can be seen that the production of particleboard with a prior art manually controlled manufacturing system does not provide efficient operation. Such inefficient operation increases the cost of manufacturing particleboard in that it requires more time than should be necessary to produce a given quantity of mats. Further, this inefficient operation increases manufacturing costs in that a considerable amount of time, effort, and equipment may be necessary if the material within the rejected mats is to be salvaged.
There is yet one other practice in the prior art particleboard manufacturing process that contributes to the overall inefficiency of a particleboard manufacturing system. This practice is the method of determining the weight of each mat by weighing both the mat and the caul upon which the mat is formed and simply deducing a nominal caul weight to determine the weight of the mat.
Since the cauls wear rather rapidly during usage, the weight of each caul is continually being reduced. Thus, in some cases, a mat that is actually within the desired weight range may be rejected because the weight of a particular caul does not closely correspond to the weight presently being attributed to each caul. Further, since the cauls do not each wear at the same rate, production must be periodically interrupted and the cauls calibrated to reference the weight of each caul to a standard weight (normally the lightest caul). This calibration technique not only requires an interruption in production but also causes the cauls to become a "matched set". Because of this, it has become common practice to replace all the cauls as soon as a number of cauls develop cracks or become rather worn. Thus, the overall efficiency of the particleboard manufacturing system is decreased and the cost of manufacturing particleboard is increased due to loss production time while the cauls are being calibrated, the rejection of a number of mats which actually lie within the acceptable weight range, and the replacement of the cauls when a number of the cauls are still usable.
Accordingly, it is an object of this invention to provide apparatus for determining the actual weight of each mat wherein the cauls do not require periodic weight calibration.
It is another object of this invention to provide a manufacturing system for producing particleboard wherein the manufacturing system is continuously controlled to produce mats within an acceptable weight range.
It is yet another object of this invention to provide a process controller for continuously controlling a particleboard manufacturing system wherein the actual weight of each mat is determined to enable the process controller to accurately control the weight of each mat being produced.
It is still another object of this invention to provide a process controller for continuously controlling the weight of mats being produced within a particleboard manufacturing system during a period of time in which the production rate of the system is being changed.
It is a still further object of this invention to provide a process controller for controlling the weight of the mats produced within a particleboard manufacturing system when the grade of particleboard being produced is changed from one particular grade to another.
Even further, it is an object of this invention to provide a process controller for controlling the weight of the mats produced within a particleboard manufacturing system when system operation is commenced after an interruption in the operation of the manufacturing system.
Further yet, it is an object of this invention to provide a process controller for controlling the weight of mats produced within a particleboard manufacturing system wherein the controller provides automatic mat weight control to compensate for variations in the density of wood particles employed during continued operation of the system, to compensate for variations in wood particle density caused by a period of suspended operation of the manufacturing system and further provides for automatic weight control during changes in the system production rate and during changes from the manufacture of one grade of particleboard to another grade.